The present invention relates to vinyl acetate-ethylene copolymer dispersions stabilized with a phosphate-functional surfactant, and the use of the copolymer to provide good scrub resistance when used as the polymeric binder in high pigment volume concentration (PVC) aqueous coating compositions, particularly in latex paints.
Manufacturers of aqueous coating formulations, and paint formulations in particular, formulate to balance the aspect of raw material cost against the required performance. Aqueous coating formulations comprise as the major components water, a film-forming polymeric binder, fillers and pigment.
The most expensive raw material is the pigment which for a white paint would be titanium dioxide. This provides both wet and dry opacity to the coating system. In order to obtain the maximum benefits from the coating formulation, the ratio of pigment, filler, and binder are used in relative proportions to minimize the use of the pigment and binder and maximize the amount of filler, thereby providing the most cost effective formulation. This results in coatings being formulated with high filler contents, giving pigment volume levels of 70 percent, 75 percent and even higher. At these high levels of pigment volume concentration [PVC], and low amounts of binder, the film integrity is reduced. The film integrity is often measured by a scrub resistance test method.
U.S. Pat. No. 4,277,385 discloses the use of plastic particles as fillers in a high PVC acrylic binder formulation, while maintaining good physical properties of the paint films formed.
Several different methods have been developed to improve the scrub resistance of high PVC binders. Binders using expensive raw materials, such as silanes, have been employed. WO 98/33831 discloses a high PVC binder having good scrub resistance by using a multi-stage polymer having two different Tg polymers. U.S. Pat. No. 5,201,948 discloses a paint having a high Tg acrylic binder plus a coalescing solvent.
The use of phosphate ester surfactants to form emulsions is disclosed in U.S. Pat. Nos. 3,963,688; and 4,307,007. U.S. Pat. No. 4,506,057 discloses a styrene/butadiene copolymer binder with 40 to 80 percent filler. U.S. Pat. No. 5,284,905 discloses advantages of neutralizing a latex made with a phosphate ester surfactant.
There is a need for a binder capable of providing good scrub resistance in high PVC coatings without the need for expensive ingredients such as silanes. Ideally the binder will also be a relatively inexpensive binder, such as an ethylene-vinyl acetate (EVA) copolymer. U.S. Pat. No. 5,874,498 discloses a copolymer of ethylene and vinyl acetate without coalescing solvents, used in forming latex paints having 60% PVC.
Surprisingly it has been found that a vinyl acetate-ethylene emulsion polymer stabilized with a phosphate-functional surfactant, and in particular an alcohol ether phosphate surfactant, provides improved scrub resistance in a high PVC aqueous coating composition.
The present invention is directed to an aqueous vinyl acetate-ethylene polymer dispersion comprising:
a) a polymer formed from monomers comprising ethylene and vinyl acetate;
b) at least one phosphate-functional surfactant;
c) at least one protective colloid; and
d) water.
The invention is also directed to an aqueous coating composition comprising:
a) at least 65 percent by volume of pigment, on a dry solids volume basis;
b) up to 25 percent by weight of a copolymer binder, comprising
1) a copolymer formed from ethylene and vinyl acetate monomer, and optionally other ethylenically unsaturated monomers,
2) a phosphate surfactant; and
3) 0.1 to 6.0 percent by weight of a protective colloid.
The present invention is directed to a novel vinyl acetate-ethylene (VA/E) polymer dispersion and its use in a high pigment volume concentration aqueous coating composition. The dispersion is stabilized with a phosphate-functional surfactant and a protective colloid.
The vinyl acetate-ethylene emulsion copolymer is formed from vinyl acetate and ethylene monomers in any known ratio, and by any conventional means. The ratio of the monomers is chosen to balance the properties of Tg, MFFT, hardness, scrub resistance and water resistance. The preferred amount of ethylene in the copolymer is from 5 to 25 weight percent, more preferably from 5 to 20 weight percent, and most preferably from 10 to 15 weight percent, based on the total amount of monomer. Preferably the polymer is formed from 30 to 95 percent by weight of vinyl acetate, more preferably from 75 to 95 percent, and most preferably from 85 to 90 percent. The Tg of the polymer of the invention is preferably from xe2x88x9220xc2x0 C. to +60xc2x0 C., more preferably xe2x88x9210xc2x0 C. to +25xc2x0 C., and most preferably from xe2x88x925xc2x0 C. to +10xc2x0 C.
In addition to vinyl acetate and ethylene, other ethylenically unsaturated monomers may be used to form the polymer of the present invention. Examples of said monomers include, but are not limited to, (meth)acrylates; maleates; vinyl esters; itaconates; unsaturated hydrocarbons; acrylonitrile; vinyl halides such a vinyl chloride; and mixtures thereof. Preferred monomers are (meth)acrylates, such as butyl acrylate, 2-ethylhexyl acrylate, and vinyl halides. Functional monomers may also be included at up to 10 percent by weight, and more preferably up to 5 percent by weight. Examples of suitable functional monomers are silanes, carboxylic acids, such as acrylic, methacrylic and maleic acid, as well as hydroxyl and amide functional monomers, e.g. hydroxyethylacrylate, hydroxypropylacrylate, acrylamide, N-vinyl formamide, N-vinyl acetamide and the like. Crosslinking monomers may also be present, such as N-methylol acrylamide, and the n-alkyl esters thereof. Since one objective of the invention is to produce a low-cost binder for use in high PVC coatings, the use of monomers more costly than vinyl acetate and ethylene is limited. If other monomers are used, preferably they make up less than 15 percent by weight of the total amount of monomer.
The VA/E copolymer dispersion is produced by emulsion polymerization using any means known in the art, including a batch, semi-batch, or continuous process. A preferred process is a semi-batch process, with some monomer in an initial charge, with the rest of the monomer added over a period of time. The amount of vinyl acetate in the initial charge preferably ranges from 0 to 50 percent, and more preferably from 5 to 20 percent of the total amount of vinyl acetate. Ethylene may be added at pressures of up to 80 bar. The ethylene may be added all in the initial charge, added all in a delayed addition, or a combination thereof with some added in the initial charge and the remainder added slowly in a delayed feed, as a continuous addition, by several discreet additions, or a combination thereof. Process conditions such as temperature, pressure and reaction time can be determined by those skilled in the art, with preferred conditions illustrated in the Examples herein.
The emulsion polymerization of the polymer can be accomplished by aqueous emulsion polymerization procedures known in the art. Optionally, conventional seeding procedures can be employed to aid in controlling polymerization to achieve the desired average particle size and particle size distribution.
The dispersion of the present invention is polymerized in the presence of at least one phosphate-functional surfactant. Examples of phosphate-functional surfactants include alcohol ether phosphates, phenol phosphates, block copolymer phosphate surfactants, alcohol phosphates, phosphated hydroxy ethyl cellulose, alcohol alkoxylate phosphates including monoester and diester forms such as alcohol ethoxylate phosphates, alcohol propoxylate phosphates, alkyl aryl ether phosphates such as nonyl phenol ether phosphates, and alcohol butoxylated phosphates. There may be propylated rather than ethoxylated forms, phosphated acids and alcohols, sodium alkyl phosphate, and phosphated olefins. A preferred surfactant is an alcohol ether surfactant, since these offer environmental advantages. Preferred alcohol ether phosphates contain from 4 to 70 ethylene oxide units, more preferably from 4 to 50 ethylene oxide units, and most preferably from 4 to 20 ethylene oxide units. The phosphate surfactant is added to the reactor either as part of the initial charge, continuously, or some combination thereof. Preferably at least some of the surfactant is added to the reactor as part of the initial charge, with the rest added continuously over a period of time following the initiation of polymerization. In one preferred embodiment, all of the surfactant is added as part of the initial charge.
The phosphate-functional surfactant is used in the emulsion polymerization at a level of up to 5 percent by weight, and preferably from 0.5 to 3 percent by weight, based on the total amount of monomer. Lesser amounts of the phosphate-functional surfactant may be used when one or more other surfactants are also used. Anionic, cationic, non-ionic and amphoteric surfactants may be used in conjunction with the phosphate surfactant, with a non-ionic surfactant being preferred. It has been found that the inclusion of some nonionic surfactant in the polymerization may produce a more stable coating composition in some coating formulations.
The phosphate-functional surfactant can be used in either the un-neutralized, partially neutralized, or fully neutralized form. Emulsions formed with the neutralized surfactant had better stability and scrub resistance. The surfactant can be neutralized with any base known in the art, such as with sodium hydroxide, potassium hydroxide, or ammonium hydroxide.
A protective colloid is used to stabilize the polymer of the invention, and is preferably used in high PVC aqueous coating compositions containing an alcohol ether phosphate stabilized VA/E dispersion. The protective colloid is used at levels up to 6 percent by weight, preferably from 0.1 to 3 percent by weight, and most preferably 0.5 to 2 percent by weight based on the total monomer content. Colloids useful in the present invention include, but are not limited to hydroxyalkyl celluloses such as hydroxymethyl cellulose, hydroxyethyl cellulose, and hydroxypropyl cellulose; polyvinyl alcohol, starch, polyvinyl pyrridine, polyvinyl pyrrolidone, and mixtures thereof. The useful colloids can have a wide range of molecular weights. One skilled in the art will recognize that with polyvinyl alcohol the degree of hydrolysis can be varied, and in the case of hydroxyalkyl celluloses the degree of ethoxylation may vary. A preferred protective colloid is hydroxyethyl cellulose.
Other adjuvants commonly used in an emulsion polymerization may also be present in the polymer dispersion, including, but not limited to initiators, oxidizing and reducing agents, buffers, molecular weight controllers, defoamers, biocides, and other such ingredients.
The dispersion of the present invention can be made to any solids level as known in the art. Generally the dispersion solids level as made is between 40 and 70 percent, and more preferably from 50 to 60 percent solids based on the dispersion.
In general smaller particle size dispersions were found to provide better scrub resistance. While not being bound to any particular theory, it is believed that smaller particles are better able to cover the filler particles in a high PVC coating composition, and thereby providing better resistance to physical abrasion. Preferably the average particle size is from 0.01 to 1 micrometer, more preferably less than 0.4 micrometers.
Dispersions having a pH of from 3 to 10 are preferred, and dispersions having a pH of from 5 to 9 are more preferred. It has been found that dispersions having a higher pH provided films having better scrub resistance.
The dispersion of the present invention is useful in forming aqueous coating compositions, especially for coating compositions having a high pigment volume concentration. The dispersion is useful as a binder, providing scrub resistance in coating compositions having a PVC of up to 95 volume percent. Preferably the PVC of the coating composition of the invention is above 65 volume percent, more preferably 70 to 90 volume percent, even more preferably 75 to 90 volume percent, and most preferably 80 to 90 volume percent. Pigment, or fillers useful in the coating composition of the present invention include any inert organic and inorganic pigment typically known in the art, including but not limited to, titanium oxide, calcium carbonate, calcium magnesium carbonate (Dolomite), aluminum silicate (China Clay), magnesium silicate (talc), mica, starch, plastics, clay, glass fibers, kaolin, wood meal, and mixtures thereof.
The aqueous coating composition is formulated using techniques known to those skilled in the art of coatings formulations. Generally, water, defoamer, stabilizer, pigment, filler and surfactant are combined under high shear forces, where the loose agglomerates of the pigments and fillers are separated into their primary particles. This dispersion should remain, as far as possible, as a stable state in order to prevent reflocculation or reagglomeration during the remaining additions to the mixture. Such reflocculation or reagglomerationcould affect the optical properties of the coating composition.
Additional water, latex binder, and additives such as rheology modifiers, biocides, anti-freezing agents, preservatives, surfactants, sequestering agents, coalescing agents, defoaming agents, humectants, colorants, waxes, and fungicides are added to the dispersion and the entire batch is blended and adjusted to the desired viscosity.
The coating compositions may be applied to a wide variety of materials known in the art, by means known in the art such as spraying, brushing, and rolling.
The following examples are presented to further illustrate and explain the present invention and should not be taken as limiting in any regard.